Chicken Egg Color Genetics Discussion

[The Moonshiner]

@The Moonshiner I know that both the pea comb and the rose comb are dominant but what if I breed a rose combed chicken to a pea combed chicken, what type of comb will their offspring have?

Never done that cross myself but I've always heard it produces a walnut comb. Walnut comb is best known as the comb type silkies have.
Interesting that a walnut is the combo of a pea and a rose and that with silkies they breed true.
Also interesting that you see lots of single combs pop up in silkies but never hear of peas or rose popping up.
Love to hear from someone that knows more on this/that whole subject.

 

[Kiki]

Every egg starts as white. Then they have two genes either the blue egg gene or non blue egg gene. (We'll call non blue white for conversations sake)
Yes blue is dominate to white
Of course they can have one blue and one white or two blues or two whites.
The genes for brown is separate and yes involves several different genes all of which aren't know. The different brown genes are why there's such a variety of shades of brown.
Lets go back to white and blue. We'll call it step one. You know how it works. Each chicken has two genes and pass one of their two to their offspring.
Isbars are pure for blue eggs and pass blue egg genes to all their offspring. In step one they are just as ameraucana or legbars they breed true for blue.
Step two is brown egg genes. No there is no green egg gene. Its just a matter of having blue and brown genes in the same bird. Brown genes on white make brown. Brown on blue makes green.
Isbars also carry brown genes so they pass one of their two blue genes and their brown egg genes to their offspring. They breed true to green because they get blue and brown from both parents.
If you look at olive eggs you can see how they came about. To get olive eggs or green eggs you cross in brown egg genes into a bird with blue egg genes.
Brown egg layers have two white egg genes so when you bring in the brown you bring in white. The offspring then have one blue gene and one white egg gene then also brown egg genes. If you then cross those together you can get one blue gene and one white gene (50%) two white genes (25%) or two blue genes (25%) the brown genes or at least some of them are going along for the ride.
With isbars selective breeding resulted in getting two blue genes and brown genes in the same bird so unless you cross in white egg genes they have no choice but to breed true for blue and brown so results in green eggs.
That's what olive eggs strive for. Problem with them is that when selective breeding to get the two blue genes back together you lose some of the brown genes so the eggs result in lighter then olive green. Breeders then breed back to a dark brown layer to darken their green eggs. When doing that they bring back in a white egg gene and have to start over in trying to get pure for blue again.
Hope that makes sense.

@EmmaRainboe

 

[DarJones]

Pea comb/single comb is a single allele on chromosome 1. There are at least 2 other comb variants associated with this allele that I have seen. A chicken with 1 pea comb and one single comb gene will show a fat pea comb phenotype. Two copies of pea comb give a pure bred pea comb chicken. Two copies of single comb give a single comb with sexual dimorphism, meaning that roosters have a large single comb and hens have a small single comb.

Rose comb is from having two copies of single comb on chromosome 1 and two copies of the rose comb inversion on chromosome 7. There are several modifier genes associated with comb type so it is common to see birds with rose comb and "peaks". Having comb genes on different chromosomes explains why it is common for crosses involving rose comb to have a percentage of straight comb offspring.

All eggs start out white. This is inherited from theropod ancestors and is a result of calcium carbonate in the egg shell. This is the normal white eggs such as are common from red games and other breeds that lay a slightly cream colored egg. White egg is easily covered in a cross with a brown egg layer. There is a zinc white gene that makes the normal white eggs brighter. Mediterranean breeds such as Leghorns commonly have the zinc white gene. Zinc white will override the biopath for brown eggs to the point that the eggs have only a slight tan tint. Cross a Brown Leghorn (zinc white layer) with any brown egg layer and the F1's will all produce barely detectable tan tinted eggs.

The gene for blue eggs is on chromosome 1 (whether asian blue or south American blue) and is very close to the gene for straight/pea comb. It is closely enough linked that a homozygous blue egg layer with pea comb such as Ameraucanas and some easter eggers can be selected for the blue egg gene based on having pea comb. Blue egg is dominant meaning that if just 1 copy of the gene is present the hen will lay blue eggs. Blue egg and pea comb can be separated at a rate of roughly 1 in 35 F2 chicks. This is why there are so many straight comb blue egg layers today.

Porphyrin is the brown coating on eggs. Sometimes it is mixed in as the egg shell is formed producing stippled eggs. Porphyrin is NOT a gene. It is a "biopath" meaning that there are multiple genes involved in production of the brown egg coating. Here is where it gets tricky. All extant chickens have the porphyrin biopath, but in some breeds one or more genes disable it. Brown leghorns that lay such pretty white eggs do so because their porphyrin biopath is disabled! Some breeds have 2 genes that cause over-expression of the porphyrin biopath. These genes cause breeds such as Marans to produce very dark brown eggs.

What about green eggs? These are a result of a combination of chickens that do NOT have the zinc white gene, have the normal white egg gene, have the blue egg gene, and have the porphyrin biopath turned on. Combine this with the porphyrin over-expression genes and the result is commonly called an olive egger. I have not tried to separate out the genetic combination that causes spearmint green eggs, but it will involve a modified porphyrin biopath.

I've seen one example of an extremely blue egg posted on BYC. I would speculate that this is caused by over-expression of the oocyanin (blue egg) gene. It would be interesting to see if a breed could be developed that produced intensely blue eggs.

 

[EmmaRainboe]

@EmmaRainboe

Yes?

 

[Kiki]

Yes?

Did you read this thread? It's got some nerdy stuff.

 

[nicalandia]

Did you read this thread? It's got some nerdy stuff.

This thread has many outstanding posters with very good knowledge on the subject matter.

 

[Kiki]

This thread has many outstanding posters with very good knowledge on the subject matter.

That's what I just said.

 

[EmmaRainboe]

Did you read this thread? It's got some nerdy stuff.

Yep, I read it. Not really anything I didn’t already know, but interesting stuff nonetheless.

 

[Allsfairinloveandbugs]

I read this entire thread. Combined with @The Moonshiner 's detailed posts from 2018 explaining that the blue/not blue egg genes are Completely separate from the brown egg "genes", and then @DarJones ' recent detailed explanation that the brown coating on eggs is porphyrin, and that porphyrin is Not a gene but a biopath, the lightbulb in my brain has Finally clicked on, and I understand how all the various egg colors and shades can be made!
The wealth of knowledge of those who have posted on this thread is extensive and invaluable, and I very much appreciate y'alls taking the time to explain.